Fat acceptance (sometimes size acceptance) is an anti-oppression movement opposing discrimination against people who are fat  ('sizeism') .
- thinness and health are not correlated , but public health information and commercial interests regularly conflate weight loss with getting fitter and/or healthier in a way that is cruel, shaming, and not evidence-based .
- there is no known evidence-based weight-loss regime that results in long-term weight loss, except for a small minority (less than 5%) of the population. 
- people do not have an obligation to themselves or to society to be healthy in any case, so even if body size is unhealthy for an individual, it is not therefore imperative on them to lose weight or imperative on others to try and convince them to do so .
In mainstream society
Fat people are targeted for shaming and discriminated against, in particular:
- They are subject to slurs and harassment. 
- They have difficulty getting medical care:
- carers may subject them to constant unwanted discussion of their weight and weight-loss plans
- they may have serious disorders ignored, misdiagnosed or go untreated because their weight is viewed as the root cause of all symptoms they have and losing it a universal cure
- treatment regimes, especially drugs, may only be tested on thinner people, or dosed for thinner people
- Fat women are less likely to get jobs or promotions than their thinner counterparts  
- Some companies force fat employees to pay more for health insurance premiums 
- Male jurors are more likely to mistrust/convict fat women than their thinner counterparts
- Advocacy for eliminating fat people from the population is mainstream, under the guise of "fighting the obesity epidemic" .
In geek culture
Discrimination against fat people takes some particular forms in geek culture:
- many geeks pride themselves on using science- and evidence-based approaches to life, and see being fat as an irrational choice
- some geeks interpret the findings of evolutionary biology as meaning that people they judge as unhealthy or unattractive should not reproduce or even should die in order to serve the goal of improving the gene pool
- geek women are often supposed to be "hot" in order to be welcome: thin, young and otherwise attractive
As with many anti-oppression movements, there are sometimes intersectionality problems, where the movement caters mostly to otherwise privileged fat people. Some fat acceptance arguments have been criticised on several axes:
- that the emphasis on research findings that fat people aren't especially likely to be unhealthy hurts people at the intersection of being fat and being disabled, by feeding the idea that it is acceptable to discriminate against people who are unhealthy: see Further Conversations On Body Image: Examining Health at Every Size (HAES)
- that fat white people claim that they are especially oppressed because white culture is especially anti-fat, and thus silencing people of color in fat discussions. Non-white fat women dispute that their cultures are generally more fat-accepting: Are Black People More Accepting of Overweight Women?
- But don't you realise fat is unhealthy?, and the archives of Shapely Prose in general
- The Rotund Marianne Kirby's fat acceptance blog
- Dances with Fat, Ragen Chastain's blog: "Life, liberty, and the pursuit of happiness are not size-dependent"
- The Fat Studies Reader, Edited by Esther Rothblum, Sondra Solovay, and Marilyn Wann
- Tipping the Scales of Justice, by Sondra Solovay
- Are Jurors Biased Against Fat Women: http://blogs.findlaw.com/law_and_life/2013/01/are-jurors-biased-against-fat-women.html
- Male Jurors More Likely to Find Fat Women Guilty, Study Says: http://www.huffingtonpost.com/2013/01/14/male-jurors-assume-fat-women-guilty-study-weight-discrimination_n_2464728.html
- Why Being Overweight Can Earn You a Lower Salary: http://business.time.com/2012/05/02/why-being-overweight-could-earn-you-a-lower-salary/